Effect of desynchronized inputs on compound sensory and muscle action potentials

Abstract

Stimulation of the second (S1) or third (S2) digit elicits a median sensory potential at the wrist. Similarly, a shock applied to the median (Sm) or ulnar (Su) nerve at the wrist evokes a sensory potential of the fourth digit and a muscle potential over the thenar eminence. Hence, a concomitant application of S1 and S2 or Sm and Su with varying interstimulus intervals simulates the effect of desynchronized inputs. In 10 hands, a shift in latency on the order of 1 msec between S1 and S2 or Sm and Su caused a major reduction in sensory potential by as much as 30-40% but little change in muscle action potential. A latency difference slightly less than one-half the total duration of unit discharge maximized the phase cancellation between the two components and consequently the loss of area under the waveform.

Physiologic changes of compound muscle action potentials related to voluntary contraction and muscle length in carpal tunnel syndrome

“…This result suggests that the pathologic process of carpal tunnel syndrome did not significantly affect temporal summation. On the other hand, the non-significant amplitude increment induced by muscle shortening in the CTS group suggests that spatial summation was significantly affected, and may also be related to decreased synchronization induced by conduction slowing or blocking resulting from neuropathy [14]. Amplitude was not significantly increased by muscle shortening, but was increased by contraction.…”

Cervical spondylotic myelopathy in elderly people: a high incidence of conduction block at C3-4 or C4-5

“…26 The combination of the two opposite changes helps to document the site of abnormality, which may otherwise escape detection. The enhancement of the initial positive wave at the site of conduction block can be explained using the concept of phase cancellation 31. An impulse that approaches but does not reach a recording electrode produces the initial positivity and a subsequent low amplitude negativity 32-34.…”

Conduction studies of the normal sural nerve

“…5) with decreasing summation and increasing cancellation of phases and increasing potential duration due to increasing asynchrony at longer conduction distance. 8 The decline in amplitude of the compound sensory potential is much more pronounced than that of the compound muscle action potential when the same segment of nerve is considered." This difference is due to the triphasic configuration and short duration (1 to 2 ms) of the single fiber nerve action potential, in contrast to the diphasic shape and longer duration (5 to 15 ms) of the motor unit potentiak8 This results in pronounced phase cancellation and decreased summation of the compound sensory action potential with even small increases in asynchrony.…”